Photomultiplier tubes are capable of extracting very faint light as electric signal and are therefore used in various kinds of fields. For example, a photometer analyses the components contained in a small amount of a sample by projecting light onto the sample and detecting fluorescence emitted from the sample through the transmitted light, scattering light, or the like with a photomultiplier tube. A charged particle beam device or the like projects an electron beam onto a sample and detects a small amount of secondary electrons generated from the surface of the sample using a scintillator and a photomultiplier tube in combination, thus enabling a detailed observation of the surface of the sample.
However, in recent years, photometers are required to analyze the components contained in extremely small amounts of samples, and charged particle beam devices are required to enable more detailed and clearer observation of sample surfaces. Accordingly, it is necessary to detect electric signals from photomultiplier tubes produced by very small amounts of light.
One of the techniques to detect electric signals from a photomultiplier tube which is produced by a very small amount of light is described in Patent Document 1, for example. In the conventional technique described in Patent Document 1, a light detecting circuit is connected to an anode of the PMT (photomultiplier tube). The light detecting circuit includes: an I/V converter converting current outputted from the anode into voltage signal V1: a dark current removing circuit removing dark current (dark count) from voltage signal V1 to generate voltage signal V2; a primary delay filter circuit shaping the voltage signal V2; an A-D converter converting voltage signal V3 having passed through the primary delay filter circuit into digital data; and an arithmetic processing unit (CPU) integrating the digital data to create light amount data corresponding to the amount of light that enters the PMT. The CPU increases the integration time as the value of the digital data decreases.